Improve clarity of CI errors for example docstring formatting

[pushfoo]

Enhancement request:

What should be added/changed?

The output of the example docstring tests should be clearer. At the moment, it looks like this:

FAILED tests/unit/test_example_docstrings.py::test_docstrings - AssertionError: python -m arcade.examples.bloom_defender not in arcade.examples.bloom_defender docs...

Specifically, we should look into adding at least two things:

1. Explain more clearly when a docstring is missing entirely
2. Explain when a docstring has an incorrect run command + what it should be

I am not sure how to include this in CI at the moment. Outputting the expected run command on failure may be easier.

What would it help with?

Two regular contributors have been confused by the output of the CI docstring checks. Improving legibility here would improve our development speed.

[gran4]

The file you are referencing doesn't seem to exist anymore.

[pushfoo]

Ty for pointing out the dead link & for the reminder! I've updated the issue & started a PR.

[einarf]

The output is improved a little bit now and paul added detailed into in the unit test itself.